In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxides of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature and this requires premixing of the fuel and all the combustion air before combustion takes place. The oxides of nitrogen (NOx) are commonly reduced by a method which uses two stages of fuel injection. Our UK patent no 1489339 discloses two stages of fuel injection to reduce NOx. Our International patent application no WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, ie less than the stoichiometric ratio.
The present invention is particularly concerned with gas turbine engines which have staged combustion, and more particularly concerned with the secondary fuel and air mixing duct and secondary fuel injection or tertiary fuel and air mixing duct and tertiary fuel injection.
In order to inject fuel into the secondary, or tertiary fuel and air mixing ducts, it is known to use cylindrical fuel injectors which extend across the inlet to the mixing duct as described in our copending UK patent application 9310690.4 filed May 24, 1993. This arrangement has suffered from preburning of fuel in the air in the mixing duct whereas the fuel should not burn until it is in the appropriate combustion zone. The fuel burns in the air in the mixing duct because of recirculation of the fuel and air in regions immediately downstream of the fuel injectors and due to hot gases in the combustion zone flowing upstream into the mixing duct.